This invention relates to an additive useful in the recovery of oil from subterranean reservoirs and, more particularly, is concerned with additives for improved waterflooding operations involving the use of succinimide or succinamide subunit-containing polymers containing ionic groups, which polymers are stable to high aqueous salt concentrations and which function as viscosifiers, co-surfactants, and modifiers capable of improving the performance of other surfactants and viscosifiers.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only a minor portion of the original oil in place by the so-called primary recovery methods, which methods utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been enployed in order to increase the recovery of oil from subterranean reservoirs. In these supplemental techniques, which are commonly referred to as secondary recovery operations, although in fact they may be primary or tertiary in sequence of employment, fluid is introduced into the reservoir in order to displace the oil therein to a suitable production system through which the oil may be drawn to the surface of the earth. The displacing medium may be a gas, an aqueous liquid such as fresh water or brine, an oil-miscible liquid such as butane, or a water and oil-miscible liquid such as an alcohol. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir.
It has long been recognized that various factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobility and viscosity of the injected water with respect to the oil, and the wettability characteristics of the subterranean rock surfaces influence the amount of oil recoverable by waterflooding. Thus, the addition of surfactants to the flood water may result in a lowering of the oil-water interfacial tension and/or alter the wettability of the entrapping rock. Similarly, the addition of viscosifiers to all or part of the injected water increases the viscosity of the aqueous phase, thus decreasing the mobility ratio between it and the oil, and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding processes. Thus far, most low tension waterflooding applications have employed anionic surfactants. For example, a paper by W. R. Foster entitled "A Low Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, February 1973, pages 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in graduated concentration in order to provide a maximum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
A major limitation to the use of surfactant in waterflooding operations is the tendency of the surfactant to precipitate from solution when exposed to moderate or high salt concentrations. Thus, as taught in the Foster paper, it may be necessary to precede the waterflooding medium with a slug of low-salt water to displace reservoir waters containing unacceptably high concentrations of salts. In addition to the gross precipitation of surfactant realized in high salt environments, surface active performance of the surfactant is affected adversely by salt concentrations, even concentrations below those which cause precipitation. Consequently, in aqueous environments with salt concentrations as low as 2 to 3 weight percent, it is difficult to achieve the desired low interfacial tension. Thus, even with the use of a protective low-salt slug of water prior to injection of the surfactant solution, it is difficult in some cases to achieve the desired results. A number of recent patents are directed to low tension waterflooding and surfactant systems which tolerate relatively high salinities and/or divalent metal ion concentrations. For example, U.S. Pat. No. 3,811,504--Flournoy et al, is directed to a low tension waterflooding process for use in environments exhibiting a polyvalent ion concentration of about 1500 to about 12,000 parts per million which employs a three-component surfactant system containing two anionic surfactants, one of which is an alkyl or alkylaryl sulfonate; the other anionic surfactant is an alkyl polyethoxy sulfate containing from 1 to 10 ethoxy groups and from 7 to 20 carbon atoms in the alkyl group. The nonionic surfactant may be a polyethoxylated alkyl phenol or a polyethoxylated aliphatic alcohol.
U.S. Pat. No. 3,508,612--Reisberg et al is directed to a low tension waterflooding process employing a calcium-compatible anionic-anionic surfactant system which can be employed in saline solutions containing from 0.01 to 5 molar NaCl and from about 0 to 0.1 molar CaCl.sub.2. One of the anionic surfactants employed in the Reisberg et al process is an organic sulfonate such as petroleum sulfonate having an average molecular weight within the range of 430-470, and the other surfactant is a sulfated ethoxylated alcohol. A preferred sulfated alcohol is one containing a C.sub.12 -C.sub.15 alkyl group and three ethylene oxide groups.
Another technique involving the use of calcium-compatible surfactant systems in low tension waterflooding is disclosed in U.S. Pat. No. 3,827,497--Dycus et al. In that patent, the patentees disclose a process in which a three component or a two component surfactant may be employed. The three component system comprises an organic sulfonate surfactant such as petroleum sulfonate, a polyalkylene glycol alkylether, and a salt of a sulfonate or sulfated oxyalkylated alcohol. The two component system comprises an organic sulfonate surfactant and a salt of a sulfonated oxyalkylated alcohol. These surfactant systems may be employed in a brine solution which will usually contain about 0.5-8% sodium chloride and will often contain 50-5,000 parts per million polyvalent metal ions such as calcium and magnesium ions.
In U.S. application Ser. No. 719,135 filed Aug. 31, 1976 and assigned to the assignee of this invention, there are disclosed succinimido arylsulfonate compounds as surfactants in waterflooding operations. In the materials of that application, however, the succinic functions are not repeating subunits of a polymeric species.
Notwithstanding the improvements in the art relating to improved surfactant compositions, materials which may serve to modify existing surfactants to improve their stability and efficiency in moderate to high aqueous salt environments are highly desirable.
One difficulty which often is encountered in secondary recovery operations is the relatively poor sweep efficiency of the injected displacing liquid. That is, the displacing liquid exhibits a tendency to channel through certain portions of the reservoir and to bypass other portions. Such poor sweep efficiency is usually occasioned by differences between the viscosity of the injected displacing medium and in situ reservoir oil and also by permeability variations within the reservoir. The reservoir may comprise a plurality of fairly well-defined zones of widely diverse permeabilities. The injected displacing fluid preferentially flows through the more permeable zones of the reservoir thus leading to premature breakthrough of the displacing fluid at the production well or wells.
Even when the reservoir exhibits a relatively uniform permeability throughout, a situation referred to as instability fingering may develop in those instances where the viscosity of the injected displacing fluid is significantly less than the viscosity of the in situ reservoir oil. In this situation, the less viscous displacing fluid tends to develop fingers or bulges which may be caused by points of minute heterogeneity in the reservoir. These fingers of displacing fluid tend to become extended in the direction of flow and travel at a faster rate than the remainder of the injected fluid, thus again resulting in premature breakthrough at the production system.
Various means have been proposed for improving the sweep efficiency of injected displacing fluids in waterflooding operations by increasing the viscosity of those fluids. This may be done prior to injection into the substratum, or may be performed in situ in order to avoid a reduction in injectivity at the injection site. For example, in U.S. Pat. No. 3,208,518 to Patton, there is disclosed a waterflooding process wherein the viscosity of the flooding medium is increased in situ through the use of high molecular weight polymers such as ionic polysaccarides produced by bacterial fermentation.
U.S. Pat. Nos. 3,677,961--Browning et al; 3,727,687--Clampitt, 3,215,634--Walker and 3,697,498--Browning et al disclose various cross-linked polysaccharides as being useful as viscosifiers in waterflooding operations. In addition, U.S. application Ser. No. 708,727, filed July 26, 1976, now U.S. Pat. No. 4,096,074 and assigned to the assignee of the present invention, discloses viscosifiers composed of cross-linked polysaccharides which are relatively stable to high concentrations of divalent metal ions.
None of these references disclose the novel polymers of the present invention which serve both as viscosifiers and as modifiers which improve the salt stability of waterflooding surfactants.